This invention relates to tile field of polyethylene blends.
Polyethylene is known as a general purpose thermoplastic. Billions of pounds of polyethylene are produced each year. Polyethylene is produced in two main processes. The first process is the high pressure, free radical, polymerization process. The second process is the low pressure, transition metal, polymerization process.
The high pressure, free radical, polymerization process produces a polyethylene resin that has, in general, low haze values and moderate strength values when used in film applications. This type of polyethylene resin tends to have several short and long chain branches, hence the reason it is recognized in the art as "branched polyethylene" or "low density polyethylene".
One particular subcategory of branched polyethylene is branched ethylene copolymers. In particular, branched ethylene copolymers having vinyl acetate (CH.sub.3 COOOCH.dbd.CH.sub.2) as a comonomer are especially interesting. One of the interesting properties of these ethylene-vinyl acetate copolymers is that they tend to have very low crystallinity but not a correspondingly low density. This is partly because the vinyl acetate groups in the copolymer tend to increase the density of the amorphous regions of the copolymer. This, therefore, provides a means of lowering the crystallinity of a polymer composition without necessarily lowering the density of this polymer composition by a corresponding amount.
The low pressure, transition metal, polymerization process produces a polyethylene resin that has, in general, high haze values and high strength values when used in film applications. This type of polyethylene resin tends to have a few short chain branches and essentially no long chain branches, hence the reason it is recognized in the art as "linear polyethylene" or "linear low density polyethylene".
It is desirable for a polyethylene film to possess high impact strength, high tear strength, and low haze. This is because a polyethylene film with better properties can use less polyethylene resin to accomplish a given task. For example, a polyethylene grocery sack that uses less polyethylene resin because it has better properties is more environmentally friendly than a polyethylene grocery sack that uses more polyethylene resin because its properties are not as good.
High impact strength is desirable because a polyethylene film with high impact strength is able to resist rupturing better than a polyethylene film with low impact strength, other facts being equal. High impact strength is especially desirable in polyethylene grocery sacks where higher impact strength can lead to a sack that uses less polyethylene resin.
High tear resistance is also desirable because a polyethylene film with high tear strength is able to resist tearing better than a polyethylene film with low tear strength, other facts being equal. High tear strength is also especially desirable in polyethylene grocery sacks where a tear can lead to the catastrophic failure of the sack.
Low haze is desirable because a polyethylene film with low haze is easier to see through than a polyethylene film with high haze. Low haze is especially desirable in applications like bread packaging where a consumer wishes to look at the product before buying the product.
It is difficult, however, to produce a polyethylene film that has high impact strength, high tear strength, and low haze. This is because there are several competing factors that affect these properties. For example, increasing the density of a polyethylene resin increases the haze of the resin (which is not desirable) and it also decreases the impact strength of the resin (which is not desirable) other factors being equal.
As another example, increasing the molecular weight of a polyethylene resin increases both the tear strength and the impact strength of the resin (which is desirable) but it also increases the haze of the resin (which is not desirable) other factors being equal.
As another example, broadening the molecular weight distribution (as hereafter defined) of a polyethylene resin increases the processability of the resin (which is desirable) but it also increases the haze of the resin (which is not desirable) other factors being equal. Easy processability is desired because it allows a polyethylene producer to process more polyethylene resin through his polyethylene equipment without degrading the resin, other factors being equal.
This invention provides an improved polyethylene composition having a desirable combination of properties.